IGCSE Biology glossary
Transpiration rate
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Definition
The speed at which water is lost from the leaves of a plant by evaporation through the stomata. Transpiration rate is increased by high temperature, low humidity, wind and high light intensity, and is decreased by the opposite conditions.
What to include in an exam answer
Know the four factors affecting transpiration rate and explain each: temperature (increases evaporation), humidity (reduces concentration gradient for water vapour), wind (removes humid air around leaves), light intensity (opens stomata).
Example in context
Explain why transpiration rate increases on a windy day (2 marks): Wind removes the layer of humid air surrounding the leaf surface. This maintains a steep concentration gradient for water vapour between the inside of the leaf and the outside air, increasing the rate of evaporation and diffusion of water through the stomata.
Related syllabus topic
Transport in plants